Porous films made from mixtures of blends of polyolefin materials used as thermal shutdown separators for batteries have been previously described. Depending on the types of polymeric mixtures employed, several disadvantages have been found. For example, porous films made from the use of incompatible polymers result in non-uniform porous structures, thereby limiting their utility. Other porous films employ different grades of the same or miscible polymeric materials but with thermal shutdown temperatures above 125.degree. C. This high shutdown temperature does not provide complete protection against thermal runaway in lithium-ion batteries. For example, under adiabatic conditions, the thermal runaway of a lithium-ion battery with carbon anodes begins at about 120.degree. C. Still other porous films embody mixtures having melting points less than 15.degree. C. apart. Temperature surges in excess of 15.degree. C. may cause potentially dangerous holes to appear if a lithium battery were to overheat.